This invention relates to a keyboard, and particularly to a low-profile keyboard comprising key tops, hinges and a housing, all integrally molded.
Referring to Figs. 1A and 1B, an example of the prior art keyboard will first be described. FIG. 1A is a top plan view illustrating key tops of the keyboard while FIG. 1B is a cross-sectional view taken on line 1B--1B and viewed in the direction indicated by the arrows in FIG. 1A.
In FIGS. 1A and 1B, designated at 10 is a housing of keyboard. The housing 10 has openings 11 formed at regular intervals through which the respective square-shaped key tops 2 are exposed. It is noted here that the key tops 2 are formed integrally with the housing 10 by means of respective hinges 22. Each of the key tops 2 is formed with engagement flaps designated at 21 on its opposite sides adjacent the portions where the hinges 22 are formed. Indicated at 3 is an elastic sheet. Shown at 31 are raised portions embossed in the form of a convex dome on the rubber sheet 3. Each of the raised portions 31 has a protrusion 32 extending from the undersurface thereof. 4 is a movable-contact sheet having movable contacts 41 formed on the undersurface thereof in correspondence to the respective key tops 2. 5 is a fixed-contact sheet bonded to the upper surface of a substrate 8 supporting the entire keyboard. The fixed-contact sheet 5 has fixed contacts 51 formed thereon in opposed relation with the respective movable contacts 41. The opposed fixed contacts and movable contacts constitute contact pairs 71. 6 is a separator formed of synthetic resin sheet having apertures 61 stamped out therethrough to define void spaces in which the respective movable contacts 41 and fixed contacts are opposed to each other. The movable-contact sheet 4, the fixed-contact sheet 5, and the separator 6 together constitute a membrane switch (sheet) 7.
With the keyboard shown in FIGS. 1A and 1B, upon being pressed down, the key top 2 is displaced downwardly about the hinges 22 to cause the protrusion 32 to elastically deform the associated raised portion 31 on the rubber sheet 3 downwardly, which in turn elastically deforms the associated movable contact 41 on the movable-contact sheet 4 downwardly into contact with the fixed contact 51 on the fixed-contact sheet 5 to thereby close the associated contact pair. Releasing the pressure on the key top 2 allows the raised portion 31 to return to its original position to open the contact pair 71.
In the keyboard as shown in FIGS. 1A and 1B, it may be desirable to reduce the width dimension of the keyboard as well as making the thickness extremely thin. In this case, by integrally molding the housing 10, key tops 2 and hinges 22 of highly rigid synthetic resin it is possible to construct a low-profile keyboard in which the entire keyboard including the housing 10, rubber sheet 3, membrane switch 7 and substrate 8 is, say 0.2 cm in thickness (height) and 10 cm.times.25 cm in depth and width.
In this regard, it should be noted that synthetic resin material having high rigidity is generally mechanically fragile. When the housing 10, key tops 2 and hinges 22 are integrally molded of such material, the hinges 22 will fracture by fatigue due to operations of the key tops 2 repeated frequently in a short period of time, resulting in shortening the useful life of the keyboard. It is hard to say that the synthetic resin material is suitable in reducing the width and depth dimensions of the keyboard since the length of the hinges 22 is compelled to be relatively long because of the mechanical fragility as well as high rigidity of the synthetic resin material of which the hinges 22 are formed.